The present invention concerns a device for measuring translation, rotation or velocity via interference of light beams diffracted by diffraction gratings which are substantially parallel to each other.
European application 0 672 891 discloses a device for measuring relative displacements between a head unit and a scale. This device is of the type where all diffraction gratings have the same spatial period or pitch P. The head unit has a light-emitting element (source), a cylindrical lens to condense the light beam provided by the source and a first diffraction grating used in transmission for splitting the light beam. The resulting diffracted beams fall onto a second grating arranged on the scale where they are diffracted in reflexion. The head unit further comprises a third grating used in transmission for mixing the diffracted beams coming back from the scale and a light-receiving element (photodetector). In all embodiments, the source and the photodetector are spatially separated respectively from the first and third gratings so that the head unit has relatively large dimensions. The distance between the mixing grating and the photodetector is actually needed because there is a plurality of interfering beams coming out of this mixing grating. Further, it is to be noted that for each diffraction event, at least one diffracted beam is not used. The unused diffracted beams represent a loss of light power, generate noise, and may lead to spurious interferences. The efficiency of such a measuring device is thus relatively low.
U.S. Pat. No. 5,424,833 discloses a measuring device of another type wherein the first and third gratings are replaced by an unique index grating used in transmission with a pitch twice as large as the pitch of the scale grating. Thus, the scale grating, which is longer than the index grating, has a pitch or spatial period smaller than that of this index grating. Further, all embodiments in this document are arranged so that the incident beam falling on the index grating has a main propagating direction comprised in a plane perpendicular to the moving direction of the scale grating and thus parallel to the lines of both gratings. In order to spatially separate the light source and the photodetector, this document proposes, in a first embodiment, to have said main propagating direction oblique relative to the direction perpendicular to the index grating in said perpendicular plane. In a second embodiment, the incident beam falls perpendicularly onto the index grating and a beam splitter is used which deflects the interference beam coming back normally from the index grating into a direction different from the light source. The first embodiment needs an extended space in a direction perpendicular to the moving direction (measurement direction) and to the direction perpendicular to the gratings. The second embodiment has the following drawbacks: it needs an extended space between the source and the index grating, it is less efficient, and it involves more parts.
European application 0 603 905 discloses a measuring device wherein two gratings are formed on the scale, a first one for splitting the light beam coming from the source and a second one with a pitch twice smaller for interchanging the directions of the two used beams diffracted by the first grating. The mixing grating used in transmission is attached to the photodetector. This arrangement is not very efficient because its resolution is twice as small as the resolution of the device of U.S. Pat. No. 5,424,833 for gratings having pitches identical to those of the latter. Further, the scale is transparent and either its two main surfaces are arranged for diffracting and/or reflecting light beams, or an additional mirror is needed. The scale is thus relatively difficult to manufacture.
An object of the invention is to provide an optical device for measuring relative movements which has great measuring accuracy while remaining of relatively simple construction.
Another object of the invention is to provide such a measuring device the arrangement of whose various parts, in particular the scale or longer grating, can be made within relatively large manufacturing tolerances without adversely affecting the accuracy of measurements.
Another object of the invention is to provide a measuring device of this type wherein the variation in wavelength of the source and of its angular spectrum have no influence on the accuracy of measurements.
Another object of the invention is to provide a device of this type allowing a very flat arrangement which can easily be miniaturised.
A particular object of the invention is to provide a device of this type at least partially integrated in a silicon or semiconductor substrate.
The invention therefore concerns a device for measuring translation, rotation or velocity via light diffraction including a light source, at least one light detector, a first grating or first and fourth gratings of the same spatial period and located substantially in a same first plane, and a second grating or second and third gratings of the same spatial period and located substantially in a same second plane; the first and, where appropriate, fourth gratings being mobile along a given direction of displacement relative to the second and, where appropriate, third gratings, this device being arranged so that a first light beam generated by said source defines a beam incident upon said first grating where this incident beam is diffracted into at least a second beam and a third beam; so that these second and third beams then reach at least partially said second grating or, where appropriate, said second and third gratings respectively, where they are respectively diffracted into at least fourth and fifth beams whose propagating directions are interchanged respectively with the propagating directions of said second and third beams; so that these fourth and fifth beams then reach at least partially said first grating or, where appropriate, said fourth grating where they are respectively diffracted in a same output diffraction direction so that they interfere, said light detector being arranged to detect at least partially light resulting from said interference; the first, second and, where appropriate, third and/or fourth gratings being used in reflexion.
The features of this measuring device allows an easy miniaturisation and its integration by microelectronic and microsystem technologies.
According to a preferred embodiment, said first and, where appropriate, fourth gratings belong to a portion of the device which is mobile relative to said incident beam, said second and, where appropriate, third gratings being fixed relative to this incident beam.
According to a particular embodiment, the first and, where appropriate, fourth gratings have a pitch or spatial period which is twice as large as that of the second and, where appropriate, third gratings, said second and third beams being diffracted respectively into the  less than  less than +1 greater than  greater than  and  less than  less than xe2x88x921 greater than  greater than  orders, said fourth and fifth beams being diffracted respectively into the  less than  less than xe2x88x921 greater than  greater than  and  less than  less than +1 greater than  greater than  orders and these fourth and fifth beams being respectively diffracted into the  less than  less than +1 greater than  greater than  and  less than  less than xe2x88x921 greater than  greater than  orders in said same output diffraction direction by said first or, where appropriate, fourth grating.
According to a preferred feature of the measuring device according to the invention, the light from said incident beam forming said second, third, fourth and fifth beams and finally detected by the detector reaches said first grating at an angle of incidence which is not zero in a plane perpendicular to lines forming the gratings, this angle of incidence being sufficient so that the light source providing said light and the detection region of the detector receiving said light are spatially separated from each other in projection in a plane perpendicular to said lines.
According to a particular feature, said output diffraction direction defines an angle, in said plane perpendicular to lines forming the gratings, which has a value substantially equal to the angle of incidence of the incident beam multiplied by  less than  less than xe2x88x921 greater than  greater than  relatively to an axis perpendicular to said gratings, only light interfering along this output diffraction direction being used for measuring a displacement. Thus, the optical arrangement is fully symmetrical and so reciprocal.